Continuous solution polymerization processes generally involve the addition of catalyst to a monomer and solvent mixture. The mixture may be back-mixed giving a uniform polymer in an environment with substantially no concentration gradients. WO 94/00500 (Pannell, et al.) describes a solution polymerization using metallocene in a continuous stirred tank reactor, which may be in a series reactor arrangement to make a variety of products.
For the purposes of this patent specification the term “metallocene” is herein defined to contain one or more cyclopentadienyl moiety in combination with a transition metal of the Periodic Table of Elements.
The heat of the polymerization reaction can be absorbed by the polymerization mixture, causing an exotherm. Alternatively, or in addition, the heat of reaction can be removed by a cooling system, by external cooling of the walls of the reactor vessel, or by internally arranged heat exchange surfaces cooled by a heat exchange fluid.
In the course of the polymerization, typically, a predominant amount (over 50 mol %) of the monomer is consumed and the polymer formed is dissolved in the solvent. The higher the concentration of the polymer, the higher the viscosity of the polymerization reaction mixture containing the polymer, solvent, and unreacted components. The mixture passes from the polymerization reactor to a finishing section in which polymer, solvent and unreacted monomer are separated. In the course of finishing, solvent and unreacted monomer are progressively removed from the polymerization mixture until the polymer can be formed into a solid pellet or bale. The separated solvent and monomer can be recycled to the polymerization reactor.
It is well known from extensive literature sources that polymer solutions can undergo phase separation at the lower critical solution temperature, with phase separation being encouraged by higher temperatures and/or by lower pressures. Solvents selection also influences the conditions where phase separation occurs.
The phenomenon of phase separation is firstly a consideration in the selection of the polymerization solvent. Appropriate polymerization monomer conversions, especially of the volatile monomers, temperatures, and pressures have to be selected for given polymer/solvent combination conditions to avoid unwanted phase separation inside the reactor. Solvents such as hexane may require an elevated pressure in excess of 50 bar to avoid two-phase conditions for olefin polymerization; solvents such as octane can maintain homogeneous one-phase conditions at lower pressures.
The phenomenon of phase separation can secondly be exploited after the reaction step to separate volatile solvent and unreacted monomer components on the one hand, and polymer on the other hand. In that case, separation at temperature well above the lower critical solution temperature is encouraged to allow the polymer to form a concentrated phase. Some earlier articles explain the general principles of which we are aware are: “A low-energy Solvent Separation Method,” by T. G. Gutowski et al, Polymer Engineering; “Solvents” by C. A. Irani et al. in Journal of Applied Polymer Science Vol 31, 1879-1899 (1986); “Separating Polymer Solutions with Supercritical Fluids,” by Mark A. McHugh et al in Macromolecules 1985, 18, 674-680; “Critical dynamics and phase separation kinetics etc,” by Hajime Tanaka in Journal of Chemical Physics 100 (7) 1 Apr. 1994 p 5323-5337; “Short Chain Branching Effect on the Cloud Point Pressures of Ethylene Copolymers etc.,” by S. J. Han et al. in Macromolecules 1998, 31, 2533-2538.
U.S. Pat. No. 3,726,843 described a process for making EPDM. Liquid phase separation has also been exploited to remove solvent from the polymerized mixture exiting from the polymerization reactor in Mitsui EP-552945-A (U.S. Pat. No. 5,599,885), which shows a continuous solution polymerization process with a metallocene catalyst. Hydrogen is added in the examples to avoid higher molecular weights at the low operating temperature. The pressure and temperature are raised to permit a subsequent pressure drop, that leads to the formation of separate lean and concentrated phases. Catalyst emerging from the reactor is recycled.
EP-552945-A does not disclose that the polymerization process may be conducted at elevated pressures to provide a wide range of polymers and outputs in the same plant arrangement. EP-552945-A uses an auto-refrigerated reactor in which the solvent is allowed to boil which favors low pressure operation. EP-552945-A does not suggest exploiting the initial elevated pressure in the finishing section.
While the single site, metallocene catalysts have a high activity; that activity is often sustained under conditions in which phase separation would occur at elevated temperatures. Continued polymerization activity during phase separation may influence polymer characteristics undesirably.
The use of single site catalysts is associated with poor solubility in the aliphatic hydrocarbon, saturated, non-polar solvents used for homogeneous solution polymerization. As a result, an aromatic catalyst solvent, such as toluene may have to be used. This in turn can complicate solvent separation to prevent toluene build up in the reactor, and lead to environmental pollution and added maintenance expenditure. EP-552945-A tries to avoid the use of toluene by slurrying the catalyst, comprising alumoxane as activator, in the polymerization solvent.
In some solution processes (see WO 98/02471 Kolthammer) the polymerized mixture is flashed off in two stages, whereby the solvent and unreacted monomer are converted to a vapor phase. Efficient extraction of solvent, etc., requires low vapor pressures and vapor phase compression or condensation followed by pumping for subsequent separation stages. Pumping is used to convey polymer from flash separation stages to a final devolatilizing extruder
U.S. Pat. No. 3,912,698 uses a heat exchanger for a liquid recycle stream to permit an increase in reactor capacity while reducing fouling in the context of a multiple flash to remove volatiles.
The use of single site catalysts is also associated with the generation of hydrogen through beta-hydride abstraction. Such hydrogen, when recycled back to reactor feed, can act as a modifier to reduce the molecular weight of the polymer. The amount of hydrogen established in polymerization may have to be increased or decreased depending on the target molecular weight.
In solution plants, solvent selection, operating temperatures, and purification systems have to be designed for a particular operating window for the desired polymerization process. Metallocene catalysts permit a wide variety of polymers to be made in terms of comonomer content, molecular weight, etc. Optimum production performance for a given type of polymer may be obtained with a particular metallocene within a specific operating window. Different types of polymer may then have to be produced in different plant lay-outs. There is, therefore, a need for a plant design that can be used more flexibly for different types of polymers and metallocene catalysts, and which also can be adapted more easily to evolving metallocene catalyst technologies than current designs of solution polymerization plants.
There is also a need for a plant design that permits more extensive molecular weight control through control of the hydrogen levels. There is an special need for such control that is compatible with series reactor operation that permits well separated split-operating conditions between the first and second reactor (one which permits feeding very low levels of hydrogen to one of the two reactors, while feeding large amounts of hydrogen to the other reactor).
There remains a need for an improved continuous solution process and plant which provides one or more of the following benefits: producing polymer economically across a broad range of operating windows including varying polymerization temperatures; producing a broad spectrum of polymers, particularly polymers of widely varying average molecular weights, molecular weight distributions, and/or comonomer contents; permitting the production of polymers having useful molecular weights at high temperatures (above 150° C.); accommodating a broad range of catalyst performance; reducing energy consumption, especially in finishing, and reducing environmental discharge; and reducing or avoiding fouling in the recycle and purification systems while using highly active metallocene type catalysts with unreacted monomer and temperature during separation processes.
It would be particularly useful to provide a process and plant which can adjust the process window to optimize performance for a given polymer type and catalyst, such that metallocene catalyst can be used to perform at a high activity within that window; while at the same time permitting a broad range of optimized performance windows for different polymer types and catalysts. It would also be beneficial to provide a process and plant which could facilitate operating at such high catalyst activities in the same finishing equipment, which can be used in a largely closed system with substantial recycling of all non-polar solvent and monomer components; with minimal contamination and minimal need to eliminate polar impurities contained in such non-polar recycle, however derived (catalyst residue; scavenger, etc), using a simple removal technique, and without using a stripping agent such as water which would contaminate the recycle.
For additional background, see also WO 94/00500 and WO92/14766.